Bandpass filters are employed in a wide variety of optical systems. Dielectric filters have been widely used because of their relatively low cost and compact construction. However, they typically have a maximum transmission efficiency for the pass wavelength of about 65%. Their pass band is not very sharp, and when sharp edges are required, two dielectric filters are typically used in series, reducing the total throughput to about 42%.
As an alternative, U.S. Pat. No. 4,669,811 discloses the use of a reflective holographic optical element (HOE) associated with a second reflector. Light to be filtered intersects the HOE at an angle so that light of the desired pass wavelength is reflected and the unwanted wavelengths pass through the HOE. The reflected light is returned to the HOE at a slightly different angle by the second reflector to produce an exiting beam. Because of the change in incidence angle, the second reflection by the HOE has a spectral pass band that is shifted relative to the first reflection by the HOE, but a relatively small amount so that the pass bands of the two reflections overlap to achieve a sharper overall pass band. The pass band of the HOE has sidebands, however, that would attenuate but not eliminate some of the undesired wavelengths that are outside the central pass band of the filter.
As another alternative, U.S. Pat. No. 5,124,859 discloses a bandpass filter comprising a container filled with a pressurized gas that has a ground state transition corresponding to the pass wavelength and that is transparent to other wavelengths. This filter concept can work with light incident over a range of angles and is used in a reflection configuration to separate the pass wavelength from other wavelengths. Operation of this filter concept is limited, however, to those wavelengths for which there is a gas with a suitable ground state transition.